I'm seeing a solid red LED on GigaBlox Nano with a 5 meter cable

– Copied from customer email –

I have a question regarding the GigaBlox Nano. You mentioned that it uses transformerless Ethernet. My understanding is that with transformerless Ethernet, when connecting a long LAN cable, such as 5 meters, there may be voltage drops that could lead to communication issues. Is that correct?

I tried connecting a 5-meter LAN cable, and the red LED is constantly lit. Could this be related to the transformerless design, or is there another possible issue?

A solid red LED indicates a link is active, but no traffic is flowing. This is expected behaviour if you have a device connected and there isn’t any data being sent/received. You might expect to see the red LED flash occasionally, which indicates traffic. If you started a stream of traffic, you should see the red LED flash more frequently to indicate traffic flow.

A red colour indicates that the connection is a 10/100Mbps connection, whereas a green colour indicates that the connection is a 1000Mbps. Again, whether you have a 10/100M link, or a 1000M link depends on what is connected.

If you are connected to a 100M only device, then you would expect to see a red colour on GigaBlox Nano. If you are connected to a 1000M capable device, then you would expect to see a green colour on GigaBlox Nano.

I can’t really debug without knowing what you have connected to GigaBlox Nano. Is it a 10/100M device, or a 1000M device? If it’s a 1000M device but you see a red LED, then it can be a cabling issue. In that case we’d start to look at the type of cable you are using.

Please share what device is connected, and we can debug from there.

Thank you for your response. It was very helpful.

First, I mentioned earlier that I had connected a 5m LAN cable, but to clarify, it was actually a 10m cable that resulted in the red LED constantly being lit.

With that clarification, I would like to confirm: when you mentioned “it can be a cabling issue,” is this referring to voltage drop due to the length of the cable? The device I am connecting is a communication device, and it seems that it might not be receiving sufficient power. Additionally, when I used 5m and 3m LAN cables, the connection worked fine, and the green LED flashed as expected.

Thanks for the information.

Regarding “a cabling issue”, it can actually be many things. On a fundamental level, if a cable is too long, or the cable is bad quality, it will cause distortion to the signal. The simplest form of distortion is linear voltage drop. For example. putting a 2V peak-to-peak signal into a 100 meter cable, you might get 1V peak-to-peak signal at the other end (just an example, it really depends on the quality of the cable).

At the most fundamental level, signal integrity refers to whether a receiver can adequately differentiate between a logic 0 and logic 1 voltage level (I’m assuming 2 voltage levels, many communication protocols have more than that, but let’s keep this simple).

There are other forms of distortion, such as voltage reflections, increase of rise/fall time, both of which are frequency dependent. I won’t go too much into the physics of it, but generally, using long cable, or using a short, poor quality cable, will affect the integrity of the signal negatively. You can also have problems due to nearby devices generating EMI, which will induce noise in the cable, further reducing signal integrity.

When signal integrity is too poor for gigabit ethernet (1000BASE-T), the switch will automatically downgrade to a slower speed (100BASE-T); slower signals are less susceptible to signal integrity issues.

Anyway, when I say “it’s a cabling issue” it could be one of many factors. In your case, it seems the issue is related to length, so a 10 meter cable isn’t working, and it seems your device is a gigabit device, since it works at 3 and 10 meters.

The specification for standard transformer based ethernet states 100 meters can be achieved using CAT6 cabling for 1000BASE-T (gigabit ethernet). Using transformerless ethernet reduces that maximum distance quite a lot.

The question then becomes “what is the maximum distance I can achieve with transformerless ethernet”. This is really a “how long is a piece of string” question. It depends on the cable type you use, it depends whether there are noisy devices nearby, and it depends on whether the two connected devices share a ground or not.

Intuitively, I wouldn’t suggest using transformerless ethernet for connections exceeding 1 meter or so. Many of our customers have very short connections to make, within a system which shares a single ground; in such situations, transformerless allows us to save space.

So, it may well be that you’re just hitting the limits of the technology by trying to get transformerless ethernet working at 10 meters at 1000BASE-T. That being said, let me know what type of cable you are using (is it a CAT5/6/7 cable? Is it shielded), and let me know if you are using anything else in the signal chain (any adapters), and I can give a better idea of whether it should work. Also let me know if there are noisy EMI sources nearby, and let me know if the connected device shares the same ground as GigaBlox Nano.

Thank you very much for the detailed explanation, it was very informative.

I connected the 10m LAN cable for testing purposes, but in actual operation, we do not need such a long distance. Since it works up to 5m and you mentioned that using it beyond 1m is not recommended, I will consider this and decide on the operation method accordingly. I believe all devices should be able to connect within 1m.

Thank you again for your response, and you can close this thread now.

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You’re welcome Hayato. Transformerless is really designed for backplane applications. Using it on cables is possible but one can’t achieve the same ranges as standard ethernet that uses transformers.

Let me know if you have any other questions or issues, thanks!

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